1. Field
The subject matter disclosed herein relates to electronic devices, and more particularly to methods and apparatuses for use in or with a device capable of receiving an SPS signal.
2. Information
The Global Positioning System (GPS), as well as other Global Navigation Satellite Systems (GNSS), rely on measurement of propagation delays of signals transmitted from space vehicle (SV) transmitters to receivers (e.g., ground-based navigation receivers). By measuring such a propagation delay, a receiver may obtain a pseudorange measurement to an associated transmitting SV. By obtaining such pseudorange measurements to four or more SVs at known orbital positions relative to the earth, a receiver may compute an estimated location of the receiver as part of a navigation solution, for example.
By way of example, certain GPS signals are modulated by a data signal comprising 30.0 second sequential data frames. Each frame includes five six-second sub-frames. Each sub-frame includes a 62-bit preamble followed by clock correction data, ephemeris data and/or almanac data. The first word in the 62-bit preamble is a TLM (telemetry word) which contains information about the age of the ephemeris data. The next word in the 62-bit preamble is a HOW (hand over word), which contains the number of counted z-epochs. These data contain the time since last “restart” of the GPS time on the previous Sunday 0:00 o'clock.
By way of example, FIG. 1A shows a structure of a sub-frame of a GPS signal, and FIG. 1B shows contents of the TLM and HOW. As illustrated, the preamble is a 62-bit sequence containing two bits (e.g., “00”) from the previous sub-frame, alone with the 30-bit TLM and 30-bit HOW.
Obtaining pseudorange measurements by measuring a propagation delay, as described above, may rely on having an accurate clock synchronized to GPS time. If a time uncertainty is less than ±three seconds (e.g., for GPS), which is the duration of a sub-frame in this example, a receiver may obtain an accurate indication of time by detecting the position of a 62-bit preamble of a sub-frame within a demodulated bit stream. For example, detection of the bit position of a 62-bit preamble may be implemented by indication of an exact match of a known 62-bit sequence including the TLM and HOW in a demodulated bit stream. If there is even a single bit corrupted in the 62-bit sequence in the preamble, however, finding an exact match between the known and/or predicted bit sequence and a portion of the preamble may be not possible. Unfortunately, such corruption of bits in a 62-bit sub-frame preamble may not be uncommon in low signal-to-noise environments.
Hence, there is a need for techniques which may provide for or otherwise support reliable, accurate, and/or otherwise robust time-setting in receivers and/or other like devices that receive Satellite Positioning System (SPS) signals.